May be for windows, or with a closed version of winrar ?For linux, this xpi is special and cannot be open (not a "normal" xpi).Could someone confirm he can open {972ce4c6-7e08-4474-a285-3208198ce6fd}.xpi on linux ?If yes, please run this and report the exact file type.

it contains a manifest and install.rdf, investigating too much in it as its a default firefox file you should not touch.not linux but i could open the xpi in windows (from "firefox-57.0.1.tar.bz2"), not the same content.

when it may be special under linux it is for security reason. they packed the omni.ja for testing purpose with an unknown (own) lz4 algorithm but dropped it. JA was JAR before and JAR = special form of ZIP = able to open for malware.

{972ce4c6-7e08-4474-a285-3208198ce6fd}.xpi is totally useless in Linux, and I remove it.If you read this , it seems some windows users wonder why this theme file seems so "special".To satisfy my curiosity.

i read too much of such trash sites - and all suggest spyhunter as THE solution.freefixer/threatfixer are two of 1000++ not valuable sites selling spyhunter with lies.

i can not tell you if that file was target for malware in the past - it never happend to me. but running the next update firefox will restore this file again - weather it is used or deletable.firefox also notice if it was modified (installation files) - in that case it will load the complete installation setup (for the update) instead the difference as archive.